Homologous recombination plays a critical role in reductional segregation of chromosomes in meiosis. Meiotic recombination is initiated by the programmed induction of DNA double strand breaks (DSBs) and involves a mechanism related to recombinational repair of DSBs in mitotic cells. The key difference between meiotic and mitotic recombination is that the meiotic process must occur between homologous chromatids and the mitotic process usually occurs between sister chromatids. The central step of recombination is homologous strand invasion and exchange. This process is catalyzed by recombinases that are structurally and functionally related to the bacterial strand exchange protein, RecA. There are two RecA-like recombinases in most eukaryotic organisms, including budding yeast and humans;Rad51 is the only RecA- like recombinase involved in mitotic recombination. Dmc1 is a meiosis-specific recombinase that can function in the absence of Rad51, but often cooperates with it. In order to promote strand invasion, recombinases must first polymerize into helical filaments on the tracts of single strand DMA (ssDNA) that form at sites of DSBs. Accessory factors, called mediators, allow recombinase to displace single strand DMA binding proteins as they form filaments. Recent evidence indicates that a second type of accessory factor expends energy to promote dissociation of recombinases from DMA. The proposed work seeks to elucidate meiotic recombination in budding yeast. The aims of this proposal are: 1. To determine the in vivo function of the recombinase-stimulating ATPases Tid1/Rdh54 and Rad54. 2. To determine the mechanisms through which Dmcl's accessory factors stimulate its biochemical activity. 3. To test predictions of a model in which DmcVs ability to favor recombination between homologous chromatids over that between sister chromatids reflects the rigidity of the filaments it forms and the distance these filaments project away from the axial element. 4. To characterize DMC1 interactions genetically by mapping 5 dmc1-ts suppressors and by isolating alleles of DMC1 that bypass accessory factor requirements. Defects in meiotic recombination cause chromosome non-disjunction and loss, both of which lead to birth defects and spontaneous abortion. Failure of recombinational repair in mitosis is implicated in the etiology of breast cancer and other malignancies.